Method for determining oxygen storage capacity

ABSTRACT

The oxygen storage capacity of an oxygen store associated with a catalytic converter of a combustion engine is computed by forming an integral which begins at the time of a changeover in the exposure, e.g., from rich to lean, and ends when the output signal of a post-catalytic converter lambda probe is less than a threshold value. A correction is performed to take into a consideration a time offset in the signals of the post-catalytic converter lambda probe. In particular, the time offset is measured to determine a time at which the integration should have been terminated, wherein this time is inferred retroactively.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application,Serial No. 10 2010 033 335.2, filed Aug. 4, 2010, pursuant to 35 U.S.C.119(a)-(d), the content of which is incorporated herein by reference inits entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

The invention relates to a method for determining the oxygen storagecapacity of an oxygen store is associated with a catalytic converter.

The following discussion of related art is provided to assist the readerin understanding the advantages of the invention, and is not to beconstrued as an admission that this related art is prior art to thisinvention.

The starting point is a situation where a catalytic converter isarranged downstream of an internal combustion engine in the outflowdirection of the exhaust gas, wherein an oxygen store is associated withthe catalytic converter; in particular, the oxygen store may beintegrated in the catalytic converter, but may also be provided as aseparate component. A so-called pre-catalytic converter lambda probe isarranged upstream of the catalytic converter, whereas a post-catalyticconverter lambda probe is arranged downstream of the catalyticconverter. Lambda probes measure the air-fuel ratio (in the exhaustgas).

A method for determining the oxygen uptake storage capacity will now bedescribed. To determine the oxygen removal capacity, it is onlynecessary to interchange “rich” with “lean” and vice versa, and inconjunction with the discussion of oxygen uptake, a method fordetermining the oxygen removal capacity will likewise assume a removalof oxygen.

To determine the oxygen (uptake) storage capacity, the oxygen store isinitially exposed to rich exhaust gas, in order to remove the oxygenfrom the oxygen store. The fact that the exhaust gas is rich isdetermined from signals of the pre-catalytic converter lambda probe,meaning under control of the lambda probe.

When the oxygen store is almost completely emptied (according to apredetermined criterion), a changeover immediately occurs to an exposureof the oxygen store to lean exhaust gas, so as to fill the oxygen storeagain slowly with oxygen. In this case, the air-fuel ratio “lean” isalso defined under control of the pre-catalytic converter lambda probe.

The quantity of oxygen taken up per time interval during refill is thenintegrated. If possible, the entire quantity of oxygen should bemeasured. Accordingly, the time interval for the integration starts atthe time offset changeover (which is determined by a controller andtherefore known). The time interval ends at a time when a full state ofthe oxygen store causes an (the) output signal of the post-catalyticconverter program to fall below a threshold value. As long as the oxygenstore is not completely filled, oxygen in the lean exhaust gas isremoved from the lean exhaust gas by the oxygen store, and the exhaustgas is no longer lean when reaching the post-catalytic converter probe.When the oxygen store is then full at some point in time, additionaloxygen is no longer stored, so that lean exhaust gas actually reachesthe post-catalytic converter probe. The voltage in the voltage signalthen typically falls below, for example, 0.4 V.

The method of the invention operates very well as long as the employedmeasuring devices are fully operational.

If the post-catalytic converter lambda probe has aged, it mayparticularly react with a delay. If the start of the time interval isthe time when the changeover occurs, then the measurement begins at thecorrect time, but ends with a delay due to the aging of thepost-catalytic converter lambda probe, because the time when theinterval ends is determined by the signals of just this post-catalyticconverter lambda probe.

It would therefore be desirable and advantageous to obviate prior artshortcomings and to provide an improved catalytic converter which takesinto consideration aging of the post-catalytic converter lambda probewhen determining the oxygen storage capacity.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method fordetermining oxygen storage capacity of an oxygen store associated with acatalytic converter of an internal combustion engine, includes the stepsof measuring, in an outflow direction of exhaust gas, an air-fuel ratiowith a pre-catalytic converter lambda probe arranged upstream of thecatalytic converter and with a post-catalytic converter lambda probearranged downstream of a section of the catalytic converter, initiallyexposing the oxygen store, under control of the pre-catalytic converterlambda probe, to rich exhaust gas so as to extract as much oxygen aspossible from the oxygen store or to lean exhaust gas so as to fill theoxygen store with as much oxygen as possible, thereafter changing overfrom the rich exhaust gas to lean exhaust gas so as to fill the oxygenstore with oxygen or changing over from the lean exhaust gas to richexhaust gas so as to remove oxygen from the oxygen store, integratingthe filled or removed quantity of oxygen over a time interval, startingat a first time of the changeover and ending at a second time when anoutput signal of the post-catalytic converter lambda probe is less thanor greater than a threshold value, indicating a full state or an emptystate of the oxygen store, measuring a time offset between the firsttime and a third time when the mathematical sign of a slope of theoutput signal of the post-catalytic converter lambda probe changes, andcorrecting the obtained integral with the time offset.

The method is based on the observation that the changeover in theexposure—clearly recognizable at the end of the time interval—canalready be adequately established almost directly from the output signalof the post-catalytic converter lambda probe. Ideally, a changeover inthe exposure is directly accompanied by a change in the slope (firstderivative with respect to time) of the output signal. However, a timedelay in the change would indicate aging of the probe.

The time delay (“probe delay”) remains constant during the entire time.It is then possible to determine, based on the time offset at the timeof the change, when the measurement should have ended. The measurementends with a delay of exactly the time offset.

If plenty of storage capacity is available (e.g., in a ring memory),then intermediate values of the integral can be stored during theintegration over time, and a final value can be obtained by subtractingthe time offset from the time when the time interval ended. Theintermediate value associated with the final value can hereby be used asa correction value. In this variant, the correct value for the integralis in a way stored.

However, sufficient storage capacity is not always available. If weightfor a ring memory or another high-capacity data storage device needs tobe reduced, then not all intermediate values of the integral can bestored and the value calculated for the “correct point in time” must beestimated. According to an advantageous feature of the presentinvention, the fraction of computed oxygen storage capacity, which hasbeen included in the integral during the length of the time offsetbefore the time interval ended, may be computationally estimated.Precisely this fraction is then subtracted from the integral forobtaining a correction value.

According to an advantageous feature of the present invention, theoxygen load of the oxygen store may increase continuously, so that thecorrection value can be obtained by a simple proportional calculationbased on the elapsed time.

According to another advantageous feature of the present invention, themethod of the invention may also be performed during arbitrary driving.In this case, the vehicle operator may change the exhaust gas mass flowduring the measurement or the air-fuel ratio may change due to a changein the load-rotation speed point. In this case, it is no longer correctto assume that the slope in the oxygen load is constant.

According to yet another advantageous feature of the present invention,a change in the exhaust gas mass or in the air-fuel ratio during thetime interval may be taken into consideration when estimating thefraction.

The effect of a time delay affects low pass filtering. According to anadvantageous feature of the present invention, the time offset maydetermine, or more particularly represent, a filter constant for such(digital) low pass filter. If the quantity “oxygen uptake per time” isfiltered with this digital low pass filter, then a result of thefiltering is obtained at the end of this time interval which can be usedas a slope of a straight line. The fraction of computed excess oxygenstorage capacity can be obtained by multiplying the slope with a timeoffset.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be morereadily apparent upon reading the following description of currentlypreferred exemplified embodiments of the invention with reference to theaccompanying drawing, in which:

FIG. 1 shows an arrangement adapted for use of the method according tothe invention,

FIG. 2A shows the air-fuel ratio lambda as determined with the methodaccording to the invention,

FIG. 2B shows response signals of a fully functional post-catalyticconverter lambda probe with a time delay following the exposureaccording to FIG. 2A,

FIG. 2C shows the oxygen load that would be calculated based on the tworesponse signals,

FIGS. 3A and 3B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 3D shows an exemplary exhaust gas mass flow, as adjusted by thevehicle operator, and

FIG. 3C shows a diagram corresponding to FIG. 2C for the situation ofFIG. 3D,

FIGS. 4A and 4B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 4D shows another exemplary exhaust gas mass flow, as adjusted bythe vehicle operator, and

FIG. 4C shows a diagram corresponding to FIG. 2C for the situation ofFIG. 4D,

FIGS. 5A and 5B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 5D shows a diagram corresponding to FIG. 4D,

FIG. 5E shows introduction of oxygen with digital low pass filtering,and

FIG. 5C shows a diagram corresponding to the FIGS. 2C, 3C and 4C for thesituation of FIG. 5D and FIG. 5E.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generallybe indicated by same reference numerals. These depicted embodiments areto be understood as illustrative of the invention and not as limiting inany way. It should also be understood that the figures are notnecessarily to scale and that the embodiments are sometimes illustratedby graphic symbols, phantom lines, diagrammatic representations andfragmentary views. In certain instances, details which are not necessaryfor an understanding of the present invention or which render otherdetails difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is showna schematic diagram of an internal combustion engine 1 with an exhaustgas system 2. The exhaust gas system 2 includes an exhaust gas catalyticconverter 3, which is constructed, for example, as a three-way catalyticconverter, as a NOx storage catalytic converter, or as an activeparticle filter, as well as an integrated oxygen store 4. The exhaustgas system 2 further includes a pre-catalytic converter lambda probewhich is arranged upstream of the exhaust gas catalytic converter 3 andoperates as a master probe, and a post-catalytic converter lambda probe6 which is associated with the exhaust gas catalytic converter 3 andoperates as a control probe.

In the present exemplary embodiment, the post-catalytic converter lambdaprobe 6 is arranged downstream of the exhaust gas catalytic converter 3.However, this post-catalytic converter lambda probe could also bearranged directly inside the exhaust gas catalytic converter 3, i.e.,following a partial volume of the oxygen store 4.

In the following, it will be assumed that the exhaust gas of theinternal combustion engine can be adjusted at least with a predeterminedaccuracy to a predetermined air-fuel ratio lambda.

The intent is here to determine the oxygen storage capacity of theoxygen store 4.

The oxygen storage capacity can be determined during uptake of oxygenand during removal of stored oxygen. In the following, determination ofthe oxygen storage capacity during uptake will be described.

Before the oxygen storage capacity during uptake of oxygen can bemeasured, the oxygen store 4 must first be completely emptied. To thisend, the internal combustion engine 1 is operated so that the exhaustgas reaching the catalytic converter 2 is rich; a corresponding curve 10is shown in FIG. 2A: initially, there is excess fuel in the exhaust gas.The excess fuel is combusted by removing oxygen from the oxygen store,which is then progressively emptied.

At some point, the oxygen store will be almost empty, whereafter achangeover can be made to exposure with lean exhaust gas, meaning withexhaust gas having an excess of air and hence also an excess of oxygenin relation to the fuel. The changeover from rich to lean is made at atime t₀. Such changeover causes a change in the mathematical sign of thetime derivative of the signal of the post-catalytic converter lambdaprobe 6 according to curve 12 exactly at the time t₀. If thepost-catalytic converter lambda probe 6 is not fully operational, butreacts with a time offset (“probe delay”), then for example curve 14 isapplicable, wherein the time derivative changes its mathematical signonly at a time t₁ after the time t₀.

The oxygen storage capacity during the oxygen uptake is to bedetermined, meaning from the time t₀. A prerequisite is that the exhaustgas mass flow is kept constant. Because the air-fuel ratio lambda iskept constant, a constant quantity of oxygen is taken up per timeinterval. The oxygen loading OSC therefore increases steadily with time,see curve 16.

The oxygen storage capacity OSC is generally computed with the followingformula:

${{O\; S\; C} = {\int_{t_{0}}^{t_{End}}0}},{23\left( {1 - {\lambda (t)}} \right)\ {t}*{\overset{.}{m}(t)}{t}},$

wherein the oxygen uptake is integrated over the entire time from t₀ tot_(End). If λ=cst. and {dot over (m)}=cst., then OSC increases linearlywith the end time of the integral t_(End), as also seen from curve 16.

The question now arises when the integration should be terminated.

Conventionally, the integration is terminated when the post-catalyticconverter probe measures a voltage that is smaller than a predeterminedthreshold value, for example 0.4 V. In this case, the lean exhaust gasreaches, without releasing additional oxygen, directed to thepost-catalytic converter probe, indicating that the oxygen store iscompletely full.

If the probe is fully functional, the time t₂ is exactly the correcttime. If the probe is not fully functional, then the voltage drops below0.4 V at a time which is too late, namely not before the time t₃.

If one integrates starting at the time the time derivative changes, thenfor a constant lambda and exhaust gas mass flow, the integral isindependent if the post-catalytic converter lambda probe 6 is fullyfunctional or not. The integral from t₀ to t₂ is identical to theintegral from t₁ to t₃.

The situation is different when the lambda value and the exhaust gasmass flow change during the time interval: As can be seen from FIGS. 3Ato 3D, when the exhaust gas mass flow {dot over (m)} changes accordingto curve 18, the slope is no longer constant when integrating accordingto the present formula: In the present example, the slope between thetimes t₀ and t₁ is greater than subsequently between the times t₁ and t₂and/or t₃, respectively. The “correct” value for the oxygen storagecapacity would be the value measured at point 20. If the post-catalyticconverter lambda probe 6 has aged and is not fully functional, then anoxygen storage capacity according to point 22 would be measured.

Accordingly, the measured oxygen storage capacity would be too low ifthe exhaust gas mass flow decreases in the meantime.

As shown in FIGS. 4A to 4D, when the catalytic converter is exposed toan exhaust gas mass flow according to curve 24, the “correct” oxygenstorage capacity measured at point 26 would be lower than the actualoxygen storage capacity measured at point 28.

To solve this problem, it is presently proposed to start measuring theoxygen storage capacity essentially at the time t₀ of the changeover; inthis case, one could not use the signal from the post-catalyticconverter lambda probe 6, but would have to use the signal from thepre-catalytic converter lambda probe 5.

In this case, the oxygen storage capacity measured on the agedpost-catalytic converter lambda probe would be too high, because theintegral is always measured up to the time t₃. Accordingly, the quantityoxygen taken up between the times t₂ and t₃ should be taken into accountin some way.

In a simplified embodiment, each intermediate value for the integral OSCis actually stored. Because the mathematical sign of the signal of thepost-catalytic converter lambda probe changes according to FIG. 14, thetime t₁ can be determined, and hence also the spacing t₁−t₀. The time t₃is also known, so that the time t₂=t₃−t₁+t₀ can be determined from therelationship t₃−t₂=t₁−t₀.

If the time t₂ is known, then the actual value of the oxygen storagecapacity can be inferred. In the simplest case, all intermediate valuesare stored when the quantity OSC is integrated, starting at the time t₀for several times t₁ in discrete intervals which a relatively small inrelation to the total time. By storing these intermediate values, thevalue of the integral at the time t₂ can still be determined at the timet₃, thus allowing determination of the correct value for the oxygenstorage capacity.

However, such a large quantity of data can not always be kept available.Therefore, the integral is preferably calculated until the time t₃,whereafter the magnitude of the integral at the time t₂ is calculatedbackwards. In the case of curve 24, digital low pass filtering can beperformed, with the filter constant determined by t₁−t₀. When filteringthe quantity of taken-up oxygen with the low pass filter, the curve 30is obtained. A point 32 is reached when computing the oxygen storagecapacity OSC, and a slope of a segment 36 located between the point 32and a point 38 still to be computed can be determined based on the value34 determined from the curve 30 at the time t₃.

A value ΔOSC between the point 32 and the point 38 is obtained bymultiplying the slope by t₃−t₂, or t₁−t₀. One arrives at the point 38and knows the actual oxygen storage capacity once the oxygen storagecapacity OSC has been computed from t₀ to t₃ and after subtracting thequantity ΔOSC.

While the invention has been illustrated and described in connectionwith currently preferred embodiments shown and described in detail, itis not intended to be limited to the details shown since variousmodifications and structural changes may be made without departing inany way from the spirit and scope of the present invention. Theembodiments were chosen and described in order to explain the principlesof the invention and practical application to thereby enable a personskilled in the art to best utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated.

1. A method for determining oxygen storage capacity of an oxygen storeassociated with a catalytic converter of an internal combustion engine,comprising the steps of: measuring, in an outflow direction of exhaustgas, an air-fuel ratio with a pre-catalytic converter lambda probearranged upstream of the catalytic converter and with a post-catalyticconverter lambda probe arranged downstream of a section of the catalyticconverter, initially exposing the oxygen store, under control of thepre-catalytic converter lambda probe, to rich exhaust gas so as toextract as much oxygen as possible from the oxygen store or to leanexhaust gas so as to fill the oxygen store with as much oxygen aspossible, thereafter changing over from the rich exhaust gas to leanexhaust gas so as to fill the oxygen store with oxygen or changing overfrom the lean exhaust gas to rich exhaust gas so as to remove oxygenfrom the oxygen store, integrating the filled or removed quantity ofoxygen over a time interval, starting at a first time of the changeoverand ending at a second time when an output signal of the post-catalyticconverter lambda probe is less than or greater than a threshold value,indicating a full state or an empty state of the oxygen store, measuringa time offset between the first time and a third time when themathematical sign of a slope of the output signal of the post-catalyticconverter lambda probe changes, and correcting the obtained integralwith the time offset.
 2. The method of claim 1, further comprising thesteps of storing intermediate values of the integral, subtracting thetime offset from the second time for obtaining a final time, and usingthe intermediate value of the integral associated with the final time asa correction value for the obtained integral.
 3. The method of claim 1,further comprising the steps of computationally estimating a fraction ofcomputed oxygen storage capacity contributing to the integral for theduration of the time offset before the second time, and subtracting thisfraction from the obtained integral for obtaining a correction value forthe obtained integral.
 4. The method of claim 3, wherein a change in theexhaust gas quantity occurring during the time interval is taken intoconsideration when estimating the fraction.
 5. The method of claim 3,wherein a change in the air-fuel ratio occurring during the timeinterval is taken into consideration when estimating the fraction. 6.The method of claim 4, further comprising the steps of: deriving afilter constant for a digital low pass filter from the time offset,filtering the oxygen uptake as a function of time with the low passfilter, and calculating the correction value for the obtained integralbased on a result of the filtering and the time offset.
 7. The method ofclaim 6, wherein the correction value for the obtained integral iscalculated based on a filtering result at the end of the time intervaland the time offset.
 8. The method of claim 5, further comprising thesteps of: deriving a filter constant for a digital low pass filter fromthe time offset, filtering the oxygen uptake as a function of time withthe low pass filter, and calculating the correction value for theobtained integral based on a result of the filtering and the timeoffset.
 9. The method of claim 8, wherein the correction value for theobtained integral is calculated based on a filtering result at the endof the time interval and the time offset.